wrote: Checking the web page att.com, I see the data available gradually getting smaller. How do you alter what you have used so you do not go over your limit?
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As your phone uses data the available remaining amount for your service billing period will go down. Total amount of data for billing period minus data used during period equals remaining data for period.
There is no way to alter what you have already used, all you can do is watch what you use from that point on.
You can start by always turning off data usage when you are within range of a wi-fi network. If you have Internet access at home with a wi-fi router, let your phone use that wi-fi connection for data instead of the phones 3G/4G data network.
Also check the configuration of the apps you have installed, as some can use data even when the apps not in use, and some even when the phone is not in use. Some phones (Apple I think), default to using 3G/4G for data when sleeping, instead of wi-fi.